1. Field of the Invention
The invention pertains to the demodulation of binary data signals transmitted by angle modulation of a carrier and, more particularly, an object of the invention is a method and a corresponding device for the demodulation of signals with constant envelope and continuous angle phase modulation by a train of binary symbols, that is CPM or continuous phase modulation, tolerating major frequency shifts.
Constant envelope modulations are widely used in radio communications because they are immune to non-linear distortions of amplitude. Continuous phase frequency modulation, wherein the phase variations due to a binary element can be spread over several bit periods, has valuable properties as regards spectral occupancy which is fairly low.
2. Description of the Prior Art
In a standard way, the demodulation is done either in a coherent demodulator, with a phase reference, by a discriminator, or in a non-coherent demodulator which may be of a differential type.
When the transmission channel is a so-called frequency hop type, i.e. when the frequency of the carrier varies by steps, according to a pseudo-random sequence, the steps being of a fixed duration, each step enables the transmission of a number, n.sub.b, of bits. Since the modulation applied is a continuous phase angle modulation, the imperfection of the transmission channel, the very high Doppler frequency drifts when the demodulator is on board (for example, in an aircraft), the instability of the oscillators of the reception channel etc., disturb the signal. These different phenomena of degradation result in frequency drifts.
The effect of these frequency drifts on signals with constant envelope and continuous angle phase modulation by a train of binary symbols is to add a continuous component to the useful signals processed. With a fixed threshold demodulation device, the continuous component produces a shift in the so-called "eye" diagram with respect to its nominal level, thus increasing the error rate per bit. A demodulation device, using high-pass filtering, is placed in error by sequences of identical bits, and this situation occurs with high probability in frequency hop links with short steps and, hence, few bits per step. Techniques for demodulation by differential phase or discriminator therefore cannot be applied in the presence of high frequency drifts.